1. Field of the Invention
The present invention relates to an electron gun for a color cathode ray tube, and more particularly, to an electrode unit of an electron gun for a color cathode ray tube, for forming a large-diameter electronic lens.
2. Description of the Related Art
In a general electron gun for a color cathode ray tube, spherical aberration and focusing characteristics are greatly affected by a main lens. Thus, in order to obtain good focusing characteristics, the spherical diameter of the main lens must be increased.
However, in an in-line electron gun, since three electron beam passing holes are formed at at least two electrodes constituting an electronic lens in an in-line configuration and since the diameter of a neck portion of a funnel where the electron gun is mounted is limited, it is impossible to make the diameter of an electron beam passing hole larger than the distance between centers of two adjacent electron beam passing holes (to be referred to as xe2x80x9can eccentric distancexe2x80x9d hereinafter).
An electrode unit for improving spherical aberration of the conventional main lens is disclosed in U.S. Pat. No. 4,370,592, which is shown in FIG. 1.
As shown in the drawing, burring portions 5b and 6b are formed at edges of a light emitting surface 5a of a focusing electrode 5 and a light receiving surface 6a of a final accelerating electrode 6, and large-diameter electron beam passing holes 5H and 6H having a predetermined depth are formed in the center thereof. Also, R, G and B small-diameter electron beam passing holes 5Hxe2x80x2 and 6Hxe2x80x2 through which R, G and B electron beams pass independently are formed in the large-diameter electron beam passing holes 5H and 6H.
When electron beams pass through the main lens constructed of the focusing electrode 5 and the final accelerating electrode 6, since the large-diameter electron beam passing holes 5H and 6H are non-circular, vertical and horizontal focusing components of the electron beams having passed through the small-diameter electron beam passing holes 5Hxe2x80x2 and 6Hxe2x80x2 in the center and the large-diameter electron beam passing holes 5H and 6H on both sides are different from each other. Thus, it is not possible to form uniform electron beam spots landing on the fluorescent surface. In other words, as shown in FIG. 2, the side electron beams RB and BB having passed through the large-diameter electron beam passing holes 5H and 6H of the focusing electrode 5 or the final accelerating electrode 6 are close to the burring portions 5b and 6b where a low voltage or a high voltage is horizontally distributed, and the central electron beam GB is relatively far from the burring portions 5b and 6b. Therefore, the side electron beams RB and BB are relatively strongly focused and the central electron beam GB is relatively weakly focused.
Also, since the distances between the side electron beams RB and BB and the burring portions 5b and 6b are different depending on the direction, the horizontal and vertical focusing forces for the side electron beams RB and BB are different from each other. Also, since the vertical distance between the central electron beam GB and the burring portions 5b and 6b is shorter than the horizontal distance therebetween, the central electron beam GB is applied to a focusing force which is strong in a vertical direction. Also, the central electron beam GB is applied to a divergent force in a diagonal direction of the large-diameter electron beam passing holes 5H and 6H. Thus, the side electron beams RB and BB having passed through the main lens have substantially triangular cross-sections and the central electron beam GB has a radially protruding cross-section, so that uniform electron beam cross-sections cannot be obtained throughout the entire surface of the fluorescent layer.
In particular, since the sizes of the small-diameter electron beam passing holes 5Hxe2x80x2 and 6Hxe2x80x2 are restricted by the diameter of a neck portion, there is a limit in increasing the eccentric distance between the small-diameter electron beam passing holes 5Hxe2x80x2 and 6Hxe2x80x2. Further, in order to reduce deflection current, the tendency is toward reduction in the diameter of a neck portion. Thus, the distance between the small-diameter electron beam passing holes 5Hxe2x80x2 and 6Hxe2x80x2 is reduced, which lowers spherical aberration and focusing characteristics.
An electrode unit of an electron gun for solving the above-described problem is disclosed in U.S. Pat. No. 5,414,323. As shown in FIG. 3, an electrode plate member 16 is disposed in the center of an outer electrode 11 having large-diameter electron beam passing holes, and a vertically elongated small-diameter electron beam passing hole 13 is formed in the center of the electrode plate member 16. Both side edge portions are recessed in a semi-elliptic shape so as to form side electron beam passing holes 14 and 15. According to this electrode unit, astigmatic aberration generated by the large-diameter electron beam passing hole can be eliminated by making the small-diameter electron beam vertically elongated. However, this electrode cannot easily compensate for 8-pole coma aberration of the central electron beam passing hole and for 6-pole coma aberration of side electron beam passing holes.
Another conventional large-diameter electrode unit is disclosed in U.S. Pat. No. 4,626,738. As shown in FIG. 4, this electrode includes an outer electrode 21 having a large-diameter electron beam passing hole, and an inner electrode 22 disposed within the outer electrode 21 and having polygonal small-diameter electron beam passing holes 22R, 22G and 22B. Here, aberration generated by the large-diameter electron beam passing hole can be corrected by the polygonal small-diameter electron beam passing holes 22R, 22G and 22B. However, it is not easy to fabricate the polygonal small-diameter electron beam passing holes 22R, 22G and 22B.
Another conventional large-diameter electrode unit is shown in FIG. 5, in which electron beam passing holes 32R, 32G and 32B of an inner electrode 31 are vertically elongated. However, it is difficult to fabricate an electron gun having this type of electrode due to the vertically elongated electron beam passing holes.
To solve the above problems, it is an objective of the present invention to provide an electrode of an electron gun for a color cathode ray tube which can easily correct aberration of an electronic lens formed by a large-diameter electron beam passing hole and improve focusing characteristics.
Accordingly, to achieve the above objective, there is provided an electrode unit of an electron gun for a cathode ray tube including an outer rim electrode having a large-diameter electron beam passing hole through which three electron beams pass, and an inner electrode installed within the outer rim electrode and having a central electron beam passing hole disposed at its center and side electron beam passing holes disposed at opposite sides of the central electron beam passing hole, the side electron beam passing holes having first and second curved portions in which the sides close to the central electron beam passing hole and the sides facing thereto have predetermined curvatures, and linear portions connecting the first and second curved portions, the vertical width and the horizontal width of the side electron beam passing holes being equal to each other.
In the present invention, a flange inwardly extending from the top end of the outer rim electrode to thus define the shape of the large-diameter electron beam passing hole is preferably formed. The first curved portions preferably have curvatures of an ellipse and the second curved portions have curvatures of a circle.